Spin-on coating processes are conventionally employed in semiconductor processing to deposit a variety of layers on semiconductor substrates, typically single-crystal silicon wafers used in integrated circuit manufacture. In addition, spin-on coating processes enjoy utility in processes wherein the substrate to be coated does not exhibit a high degree of crystallinity, e.g. in the manufacture of low-cost photovoltaic solar cells employing economical multi-crystalline semiconductor materials.
Coating materials conventionally applied by spin-coating processes include but are not limited to: dopant containing materials which are subjected to post deposition treatment, such as heat treatment, to diffuse a dopant into a semiconductor substrate for forming e.g. a p-n diode junction therein; materials which upon post-deposition treatment form antireflective layers; electrically conductive materials for forming transparent or opaque electrodes or contacts; dielectric materials used as insulative layers, protective coatings, and gap-fill and damascene style metallization process materials; and photoresist materials used in photolithographic or other type selective patterning processing as by chemical or physical etching.
A conventional spin-on deposition technique of, for example, a dielectric material involves preparing a fluid consisting of the coating material dissolved, dispersed, or suspended in a suitable volatile solvent or other vehicle, along with any other process or product enhancing additive; dispensing an amount of the fluid on a substrate, e.g., a semiconductor wafer; and spinning the wafer with a rotational speed sufficient to spread the coating fluid in a uniform thickness over at least the portion of the wafer intended to be coated. The rotational speed, surface tension, and viscosity of the coating fluid generally determine the thickness of the resulting coating. Following spin-on deposition, the deposited film is cured at an elevated temperature and for a time sufficient to obtain a dielectric film having the desired properties.
A typical apparatus for spin-on coating of substrates, such as semiconductor wafers, is illustrated in FIG. 1 and comprises a coating chamber, generally 10, which includes a support 11 for mounting thereon a substrate 12, typically a semiconductor wafer comprising an integrated circuit. The support 11 is mounted on a rotatable shaft 13 which is coupled to a motor 14 for spinning the support 11. The chamber 10 is coupled through outlet 15 to an exhaust system (not shown). A dispenser 16 is provided above the height of the support 11 for introducing a fluid containing a desired coating material 17 or precursor thereof on the exposed upper surface of the wafer substrate 12.
A typical operational sequence, termed a "recipe", for applying a spin-on coating such as a dielectric gap fill material (e.g., methyl siloxane-based spin-on glass (SOG) or hydrogen silsesquioxane (HSQ)) to a semiconductor wafer including integrated circuitry formed therein or thereon is illustrated in FIG. 2. As may be apparent from FIG. 2, the recipe comprises several stages, including dispensing the coating material while spinning the wafer at a low speed, accelerating the rotation of the coated substrate to spread the coating material over the substrate surface in a desired thickness, and simultaneously increasing the exhaust level of the atmosphere surrounding the coated wafer.
More particularly, and with reference to the spin coating apparatus 10 illustrated in FIG. 1, a substrate, e.g. a silicon wafer 12, is mounted for rotation on support 11. Starting at time (t)=0 and continuing for a preselected dispense time interval (t.sub.d), a preselected amount of fluidized coating material 17 is dispensed from dispenser 16 to form a mass at about the central portion of the wafer, while simultaneously rotating the substrate at a preselected relatively slow speed (r.sub.s). During the dispensing interval (t.sub.d), the atmosphere within the coating chamber 10 is maintained at ambient, i.e. atmospheric pressure. Immediately following completion of the dispense phase of the process, the spin rate is rapidly accelerated ("ramped up") over a short time interval (t.sub.a) to reach a preselected, substantially faster rotational speed (r.sub.f) which is sufficient to uniformly spread by centrifugal force the dispensed coating material 17 over at least a desired portion of the wafer 12. Concomitant with the acceleration of the wafer rotation speed, the atmosphere in the coating chamber 10 is exhausted by vacuum means (not shown) connected to exhaust port 15. The exhaust level is increased ("ramped up") over a preselected time interval (t.sub.r) to reach a preselected exhaust level (p.sub.e), and maintained at that level for the remaining time interval (t.sub.e) of the process recipe. The rotational speed of the wafer is maintained at the substantially higher speed (r.sub.f) for a preselected time interval (t.sub.s) sufficient to ensure proper spreading of the dispensed coating material, and thereafter reduced to a lower rate over a preselected time interval (t.sub.f). Finally, the coated wafer is removed from the spin-on chamber 10 and subjected to further processing, e.g. heat treatment, to develop a desired property or characteristic. In some instances, the dispensed coating material contains a precursor of the final coating material, in which instance the precursor must be subjected to additional processing in order to obtain the desired coating.
In the above-described process recipe, the principal function of exhausting of the atmosphere surrounding the coated wafer is to assist in removal by evaporation of volatile, non-coating components from the dispensed coating fluid, e.g. solvents, vehicles, flowing agents, viscosity adjusting agents, etc. However, solid components of the coating material fluid tend to become entrapped in the vapors of such evaporating components, particularly solvents, thereby forming aerosol type particulates which can redeposit on the surface of the coating layer if not removed quickly enough from the vicinity of the coating surface. The result of such aerosol particle formation and redeposition results in a coating containing surface protrusions and interior defects. Redeposition of aerosol particles is acutely problematic when depositing newer, low dielectric constant ("low k") spin-on materials, such as hydrogen silsesquioxane (HSQ), which employ volatile solvents such as methyl isobutyl ketone (MIBK). Accordingly, a need exists for spin-on methodology enabling the deposition of coatings, such as dielectric coatings, while substantially reducing or eliminating the generation of defects, particularly defects caused by redeposition of aerosol particles.